In the current speech input implementation, the application locale is used as

language in case that no language attribute is provided. For this, the
l10n_util::GetApplicationLocale was used from the IO thread. However this is
not valid since that function requires file access.

After considering different options this patch now uses the first of the accepted languages, defaulting to "en-US" in case that this list is empty.

BUG=53598
TEST=SpeechRecognitionRequestTest

Review URL: http://codereview.chromium.org/4896001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@66605 0039d316-1c4b-4281-b951-d872f2087c98